The present invention generally relates to the decontamination of soil materials containing solid organic explosives therein, and more particularly to a high efficiency process for removing solid portions of organic explosive compounds from contaminated soil using a specialized multi-step bioremediation procedure.
Environmental decontamination programs often require the removal of chemical contaminants from soil-containing regions over wide geographical areas. Of particular importance is the elimination of explosive compositions from soil materials, with particular emphasis on solid organic explosives including but not limited to trinitrotoluene ("TNT"), trimethylenetrinitronitramine ("RDX"), and tetramethylenetetranitramine ("HMX"), as well as other nitramine and nitroaromatic explosive materials. As discussed further below, the present invention shall not be restricted to the removal of any particular organic explosive materials. Likewise, the term "solid organic explosive" as used herein shall involve carbon containing compositions that are chemically capable of exploding or otherwise detonating for commercial, military, and/or other purposes. The presence of these compositions within soil materials (particularly in the form of large individual portions or "chunks") can occur at numerous locations including military installations, ordinance factories, chemical/explosive processing plants, and sites where munitions have been detonated as part of a military conflict or ordinance testing program. In particular, solid organic explosives may be present as raw waste products obtained from, for example, manufacturing plants or can also consist of unexploded (or partially exploded) munitions. Residual organic explosives can cause a number of health and safety problems if such materials are allowed to remain within the soil in an untreated state.
Regardless of the particular size parameters associated with residual organic explosives in contaminated soil materials (which will be discussed in considerable detail below), the presence of these compositions can again have serious ecotoxicological consequences. These consequences are typically manifested in many undesired effects and conditions which are generally exemplified in Won, W. D., et al., "Toxicity and Mutagenicity of 2,4,6-Trinitrotoluene and Its Microbial Metabolites", Appl. Environ. Microbiol., 31:576-580 (1976). For example, the adverse effects of organic explosive contamination include but are not limited to toxicity to algae, copepods, oyster larvae, and a generally high level of mutagenicity. A substantial interest therefore exists in the development of an effective, economical, and environmentally-compatible method for eliminating residual solid organic explosives from contaminated soil sites (particularly explosives in the form of large portions or chunks which present special difficulties as discussed below).
A number of prior decontamination methods have been developed which involve the use of microorganisms (e.g. bacteria) that are naturally present in the soil to digest, consume, and otherwise degrade various explosive materials. These bacterial processes (which are generally designated herein as "bioremediation" methods) are characterized by a number of very specific parameters, operating conditions, and procedures which will generally control the overall outcome of the treatment program being employed. For example, bioremediation systems have been used which involve conventional composting techniques. This approach is discussed in, for example, Williams, R. T., et al., "Composting of Explosives and Propellant Contaminated Soils Under Thermophilic and Mesophilic Conditions", J. Ind. Microbiol. 9:137-144 (1992). Other bacterial remediation methods which are applicable to the treatment of explosive-contaminated soil include the preparation of anaerobic bacteria slurries which are combined with the soil materials of concern as generally discussed in, for example, Funk, S. B., et al., "Initial-Phase Optimization for Bioremediation of Munition Compound-Contaminated Soil", Appl. Environ. Microbiol., 59(7):2171-2177 (1993). Likewise, aerobic bacteria slurries have also been used to remove explosives from contaminated soil as outlined in a number of references including Bradley, P. M., et al., "Factors Affecting Microbial 2,4,6-Trinitrotoluene Mineralization in Contaminated Soil", Environ. Sci. Technol., 29:802-806 (1995).
All of the techniques listed above involve a common basic procedure, namely, the use of microorganisms (e.g. bacteria) in either an anaerobic or aerobic state to treat the soil materials of interest so that residual solid explosive compositions are digested and removed by the bacteria. However, the use of composting methods offers considerable promise for a variety of reasons including an increased degree of simplicity, lower cost and labor requirements, and environmental compatibility. Composting processes are also applicable to many different explosive materials and again employ natural soil bacteria. While composting methods offer many important benefits (and are therefore the focus of the present invention), traditional composting systems normally experience a substantial loss of efficiency when the removal of large portions or "chunks" of organic explosives is required. Specifically, in standard composting systems, increases in the overall size (e.g. weight) of the explosive portions to be treated will result in decreased and often incomplete (or very slow) remediation. While it is not possible to provide exact size parameters which cause losses in composting efficiency for all situations, such problems will typically result when explosive portions are treated which weigh at least about 0.01 grams or more. Further numerical data involving size and weight parameters which cause difficulties in traditional composting systems will be presented below in the Detailed Description of Preferred Embodiments section.
As the overall size and weight of the explosive portions increases, a loss in bioremediation efficiency typically occurs because these materials are generally less "available" (or "bioavailable") to the bacteria. The terms "available", "availability", "bioavailable", and "bioavailability" as used herein involve the ability of bacteria to consume and otherwise digest the explosives under consideration which is substantially influenced by the physical parameters of the explosive portions including their size, density, degree of agglomeration, weight, and other related factors. Solid organic explosives that are present in a large chunk-like state are often too large and dense to enable effective bacterial consumption of these materials using conventional composting techniques. Thus, the bio-conversion of solid organic explosives by soil-borne bacteria occurs according to the general bioavailability of these materials which, in turn, is a function of the physical parameters of the explosive portions in question. All of these factors combine to generally define a process known as mass transfers which involves the availability and overall conversion capacity of the bacterial system(s) under consideration based on the physical state of the materials being treated.
The overall availability (e.g. bioavailability) of soil-dispersed organic explosives in bioremediation systems is typically controlled by the extent to which the explosives are dissolved within the soil. A greater degree of explosive dissolution and homogeneous dispersion of these materials will typically improve bioavailability levels and composting efficiency. For example, agitation of the soil materials being treated, as well as the use of surfactant compositions have been employed to improve the bioremediation process. The addition of surfactant compositions for this purpose is discussed in, for example, Zappi, M. E., et al., "Aerobic Treatment of Explosives-Contaminated Soils Using Two Engineering Approaches", Bioremed. Recalc. Org., pp. 281-288 (1995). According to this reference, the addition of a specific commercial surfactant sold under the trademark "TWEEN 80" at a 3% by weight amount (relative to the amount of dry contaminated soil) increased the consumption rate of waste TNT in a continuously-stirred aqueous reactor system. Similar results were achieved through the use of polycyclic aromatic hydrocarbon-based surfactants as discussed in Soeder, C. J., et al., "Influence of Phytogenic Surfactants (Quillaya Saponin and Soya Lecithin) on Bio-Elimination of Phenanthrene and Fluoranthene by Three Bacteria", Appl. Microbiol. Biotechnol., 44:654-659 (1996).
In addition to the use of biological (e.g. bacteria-based) decontamination systems, various non-biological methods have been employed in connection with explosive-contaminated soil including solvent-extraction techniques. For example, laboratory and pilot-scale tests involving the use of acetone applied to explosive-contaminated soils are discussed in a document prepared by the U.S. Environmental Protection Agency (No. EPA/625/R-93/013) entitled "Approaches for the Remediation of Federal Facility Sites Contaminated with Explosive or Radioactive Wastes", Vol. 51 (September 1993). As noted above, this particular testing program did not employ a bacteria-based composting system in which explosive materials were permitted to remain in the soil for bioremediation. Instead, the solvent (e.g. acetone) was simply used as an extractant. The acetone was applied to the soil and thereafter removed in explosive-laden form. In this type of system, the resulting product removed from the soil is a hazardous waste solvent composition containing dissolved explosives therein. Thus, a detrimental characteristic of solvent-extraction systems is the production of another waste product (e.g. the explosive-laden waste solvent) which creates substantial storage, disposal, and treatment problems. In fact, an entirely safe recovery method for the explosive-laden waste solvents that are generated during conventional extraction procedures has not yet been found, with the resulting contaminated product representing a hazardous waste material.
A final method for treating explosive-contaminated soil involves the use of physical separation methods including but not limited to the manual screening of soil materials to remove explosive portions of a selected size. Not only is this process slow and labor-intensive, but it creates additional hazardous waste (e.g. explosive chunks) materials which must be further processed.
Accordingly, prior to the development of the present invention, a need remained for a safe, effective, environmentally-conscious, and cost-efficient method for treating soil containing solid organic explosives (especially in large portions or "chunks" as further defined below.) The claimed invention satisfies this need by providing a unique process which employs a distinctive, novel, and highly-effective combination of multiple technologies. The process described below avoids (A) the generation of explosive-laden hazardous waste products which are generated by solvent-extraction methods; and (B) the difficulties associated with composting methods when large portions or chunks of explosive materials are treated, namely, the inability of soil bacteria to degrade such materials over a reasonable amount of time (if at all) due to bioavailability problems. The invention offers a number of key benefits including (1) the ability to decontaminate large quantities of organic explosive-containing soil notwithstanding the presence of explosive portions therein of a large size that are not normally treatable in an effective manner by traditional bioremediation methods; (2) a considerable improvement in the ability of the soil bacteria to metabolize the desired explosive compounds (regardless of the physical form of such materials); (3) the avoidance of any process steps which generate hazardous waste by-products that require further treatment, disposal, or storage (including contaminated explosive-laden waste solvents); (4) the capability to conduct on-site remediation at a wide variety of geographical locations using a minimal amount of equipment and labor; and (5) the general ability to effectively treat organic explosive-contaminated soil in a manner which is environmentally compatible, rapid, and cost-effective. In accordance with these benefits and the unique combination of process steps described in detail below, the present invention represents an advance in the art of soil decontamination and bioremediation technology.